Is it possible to "unsync" photos to the icloud?

With each person sharing the same Apple ID, your wife will have to turn off iCloud syncing of photos, and can then delete those photos from her phone. They will no longer appear on her iPhone when you take new pictures if she is not syncing the photos. Data is meant to be shared across devices using the same Apple ID so you can start a task on one device and continue on another. Apple recommends that each person should be using their own Apple ID. If is also likely that more data other than photos are being synced across the 2 devices using the same Apple Account.

Make sure that each family member has a unique Apple Account - Apple Support

SiporePoogie wrote: we share the icloud, all the photos I uploaded suddenly appeared on her phone's photo collection… question #1 is: is there a way to undo that?

No.

Question #2 is, if she shuts off the "iCloud Photos" in her iPhone settings, will her photos on her iphone be affected (lost)?

No, UNLESS she's using "Optimize" to reduce storage. If she's using "Optimize," then her pictures' original files may not be on her phone. Optimize saves storage by keeping the full sized pictures at iCloud.com until they are needed for editing or printing, etc.

Just curious if you don’t sync to iCloud are those photos saved to the device?